(02-20) 13:03 PST SAN FRANCISCO -- San Francisco firefighters evacuated four Mission District buildings Wednesday after contractors dropped a large rock on a plastic natural-gas main, puncturing the line and sending gas spewing into the air.

Pacific Gas and Electric Co. crews managed to cap the line at 12:35 p.m., and residents and business owners were allowed to return.

The third-party contractors were doing sewer work at 24th and Bryant streets at 10:51 a.m. when they dropped a boulder on the 2-inch gas main, said Joe Molica, a PG&E spokesman.

Utility crews heard the report of the break on their Fire Department scanner and arrived at the scene within four minutes. But it took them more than 90 minutes to cap the leak because the boulder had shattered a T-joint in the main line, Molica said.

"We had to dig three holes and squeeze this off from upstream on three pipes," Molica said.

Firefighters evacuated homes and businesses at the intersection and advised others nearby to clear the area, said Deputy Fire Chief Mark Gonzales.